Atrial fibrillation (AF) is the commonest arrhythmia for which drug therapy is currently prescribed. Many of the antiarrhythmic drugs used to suppress AF however, may cause marked QT prolongation and polymorphic ventricular tachycardia (torsades de pointes). Furthermore, this variability is not confined to antiarrhythmic drugs alone: in fact the risk of proarrhythmia related to excessive QT prolongation has been the single leading cause of drug withdrawal in the last decade. The goal of the present research is to study mechanisms underlying a well-recognized but incompletely understood behavior of the QT interval: following conversion of AF, QT interval transiently and variably prolongs and this can trigger torsades de pointes. We have accumulated evidence that following conversion of AF, the QT interval prolongs in a rate-independent fashion, and that the period following conversion of AF is one of high risk for torsades de pointes in some patients. These data support the working hypothesis in this research that AF generates signals that influence QT behavior and arrhythmia susceptibility following conversion. Such signals may be neurohormonal or rate- related and these will be evaluated in the specific experiments proposed. In Specific Aim 1, we will relate RR- QT variability prior to and following elective DC-cardioversion of AF to candidate biomarkers of the complex inflammatory, pro-oxidant, and autonomic environment mediating the clinical course of AF. In Specific Aim 2, we test the hypothesis that rapid antecedent rates, for minutes to days, contribute to RR-QT remodeling by studying the effects of atrial pacing on RR-QT relationship in patients with permanent pacemakers. Common genetic variants can modulate ion currents or the environment in which these currents accomplish normal or abnormal repolarization. Accordingly, in Specific Aim 3, the role of a set of such polymorphisms in modulating QT variability (as determined in Specific Aims 1 and 2) in AF will be determined. These studies will infer mechanisms that will be widely useful in furthering an understanding of arrhythmia risk, particularly as it applies to abnormal ventricular repolarization, and also has the potential to assist in new drug development. Project narrative: In this study, we are trying to better understand the risk factors that predispose a patient to this arrhythmia. The results of this work may help us develop new and better to treat atrial fibrillation.